Glioblastoma multiforme (GBM) is devastating brain cancer with a mean survival of only 12 months and few therapeutic options. Thus, more effective treatment is urgently needed. Ataxia telangiectasia (A-T) mutated (ATM) is a critical genome surveillance protein that regulates many DNA damage responses including cell cycle checkpoints, DNA repair, and apoptosis. It is believed that ATM also plays additional roles in regulating responses to mitogens and growth factors including insulin, and serves as a master regulator of cellular homeostasis. Because of the extreme radiosensitivity of A-T cells, inhibitors of ATM would be attractive as radiosensitizers for GBM and other types of cancers. Recently, small molecule inhibitors based on the PI3K inhibitor LY294002 were developed by KuDOS Pharmaceuticals, Ltd, that specifically target the ATM kinase. These inhibitors are effective in the nanomolar to micromolar range and radiosensitize various human tumor cell lines in vitro. We recently demonstrated that these inhibitors also suppress DNA double-strand break (DSB) repair. Herein, a second-generation derivative, KU-60019, based on the effective and extensively used predecessor KU-55933, will be tested in vitro and in vivo to determine whether it would be a safe and effective radiosensitizer for GBM. Initial experiments will use brain organotypic slice cultures to characterize the effects of KU-60019 on various radiation responses and whether normal brain with its different types of cells and the tumor cells are affected differently. Specific attention will be given to the possible adverse effects of KU-60019 on neural stem and progenitor cells. Then, the evaluation of KU-60019 as a radiosensitizer of human orthotopic GBM xenografts grown in nude mice will be determined by non-invasive bioluminescence and fluorescence imaging. We expect to determine whether KU-60019 would be a safe and effective radiosensitizer for GBM. We also expect to establish the foundation for an in vivo mouse model system that would allow us to investigate the basic radiobiological properties of neural stem and progenitor cells and assess their behavior and response to KU-60019 therapy.